Hey there! I'm happy to help!
Poverty Point is an ancient site in Louisiana with tons of man-made mounds. HUGE mounds! They were made by indigenous people. There really aren't many other places like it.
A complex society is one that innovates and improves. Lots of people work together to help make the society better and make improvements. Different people have different duties, situations, dwellings, etc. To create something as fascinating as Poverty Point must have required a complex society.
Have a wonderful day! :D